Does the book discuss how George Kennan was opposed to NATO in concept?
Kennen of ' the long telegram' was a brilliant and knowledgeable man.
But he had his blind spots.
He, for obvious reasons, viewed Russia through Stalin's time. It was Stalin who changed the Revolution to "socialism in one country". WW2 just dropped Eastern and Central Europe in his lap.

I have not read the book, so I am certain there are details in it that I would learn from.
But it must be remembered that Kennan oppos3d NATO. Partly because he didn't understand NATO,

Kennan never understood NATO either in its economic purposes nor in how it worked for the advancement of American interests.

Kennan only knew Communist Russia. He did not know todays fascist keleptocracy state.

Putin envisions a reinstated Tsarist Empire in Eurasia. He deeply resents NATO presence as it frustrates his vision for Russian expansion.
He would not, and did not, go to war with Ukraine because of this.
He invade Ukraine because it enrages him that Ukraine exists as a Nation seperate from Russia.
His view is that Ukraine is part of Russia, and he intends to make that a geopolitical reality.

Obviously the Ukrainians disagree.